This invention relates in general to data transmission and reception, and in particular to high speed voiceband data transmission and reception systems.
A modem, an acronym for modulator/demodulator, is a device that allows the transfer of data across a medium. This medium may comprise satellite links, microwave transmission systems or telephone circuits for the voiceband. The data transferred is from an information store such as a computer, which communicates to another information store through the modem link.
Conventional modems for transferring data over standard telephone voice grade lines operate at various rates from 300 baud to 4800 baud. The telephone voice grade lines allow a bandwidth of 350 Hz to 3500 Hz. The most common data transfer techniques for modems operating in the voiceband are frequency shift keying, phase shift keying and quadurature amplitude modulation. Frequency shift keying is the process of representing a computer data bit as a logical "1" at a specific frequency and a logical "0" at another frequency. As the data ("1" or "0") is transferred to the modem for transmission, the representative frequency is varied correspondingly. In phase shift keying, the two logical states "1", "0" are represented by two different phases of the signal. With frequency shift keying and phase shift keying, the data transfer rates can be as high as 4800 baud with low bit error rates (normally about one error in every million bits transferred).
In frequency shift keying, one bit is transferred at a time. In phase shift keying, one or two bits may be transferred at a time. In quadurature amplitude modulation, four bits may be transferred at a time; this allows the data rate to be increased up to 9600 baud. Quadurature amplitude modulation employs a combination of amplitude modulation and phase shift modulation to transfer four bits at a time. To further reduce telephone costs, it is desirable to further increase the data rates. It is therefore desirable to provide data transmission and reception systems which transfer data at rates higher than the systems described above.